sysutils/cpuid2cpuflags: make the port build and work on ARM

• Replace getauxval(3), a non-standard glibc extension, with our native elf_aux_info(3) call
• Return features supported by the specific machine processor architecture (uname -p) rather than the hardware platform (uname -m), because the latter is too vague on FreeBSD/arm

PR: 258445
Reported by: pkg-fallout